With a three-point seat belt, a passenger constrains his/her body by pulling a seat belt tongue held at his/her shoulder to withdraw a shoulder belt and waist belt formed of one webbing simultaneously, and inserting the seat belt tongue into a seat belt buckle provided at a seat portion opposite to the position from where the waist belt is withdrawn in a manner to constrain the upper body and waist of the passenger.
The structure from which the waist belt is withdrawn is classified into a structure configured to fix one end of the waist belt to the seat or a surrounding member directly, and a structure configured to fix one end of the waist belt engageably and disengageably to the seat or a surrounding member via a buckle. The buckle in the latter structure is called a connector buckle, and used as engaged with a connector buckle tongue provided at the one end side of the waist belt. Except for such occasions as vehicle maintenance or the like, this connector buckle is usually kept engaged with the connector buckle tongue, so that it is possible to fasten the seat belt anytime by engaging a seat belt tongue into a seat belt buckle. A buckle structure of this type is disclosed in, e.g., PTL 1.
Connector buckles are required to have a smaller and simpler structure, because connector buckles are obstacles against utilization of the space inside the vehicles. Particularly, in the back seat, a large connector buckle would touch the waist of a passenger and give an uncomfortable feeling. In a type of a vehicle in which the seat belt device can be retracted inside the vehicle when nobody is sitting, a connector buckle left on the seat would become an obstacle against space utilization such as mounting of baggage on the seat, etc.
An example of a buckle structure that has been put into practical use as a connector buckle will be described with reference to FIG. 1. FIG. 1 is an exploded perspective diagram of a structure of a conventional connector buckle.
The conventional connector buckle 100 is composed of a base member 110, a latch member 120, an unlocking member 130, a leaf spring 135, an ejector 140, a coil spring 145, a stopper member 150, and cover members 160a and 160b. 
Each member will be described. The base member 110 includes: an inner region 111 into which a tongue 101 having a locking groove 102 in a side thereof along an inserting direction I can be inserted; and slide grooves 112 formed in upper and lower facing surfaces of the base member 110, respectively. The latch member 120 is inserted through the slide grooves 112 of the base member 110 and disposed such that both ends thereof stick out from the base member 110. The unlocking member 130 is a squared-U-shaped member which, with an open mouth thereof ahead, is externally fitted over a side surface of the base member 110, and includes locking support portions 131a and 131b configured to be locked with the both ends of the latch member 120 sticking out from the base member 110 such that the unlocking member 130 is supported in the externally fitted state. The leaf spring 135 is inserted between the base member 110 and the unlocking member 130, and biased to slide the unlocking member 130 in a first sliding direction X1 of the latch member 120. The ejector 140 is provided in the inner region 111 of the base member 110. The coil spring 145 is provided between the ejector 140 and the stopper member 150, and biased to push out the ejector 140 in a releasing direction E opposite to the inserting direction I of the tongue 101. The stopper member 150 is a member having a bracket shape in a plan view, and provided on the base member 110 such that locking portions 151a and 151b thereof formed at ends thereof are locked with claws 113a and 113b of the base member 110. The upper and lower two cover members 160a and 160b house therein the base member 110 fitted with these members in the way described above in a manner to sandwich it, and are fixed in the housing state by means of a screw 161.
In the connector buckle 100 configured as described above, when the tongue 101 is inserted along the inserting direction I, the latch member 120 slides in a second sliding direction X2 once by being pushed and moved by a head of the tongue 101, and when it is at the locking groove 102 of the tongue 101, slides in the first sliding direction X1 by means of the leaf spring 135, to be thereby locked with the locking groove 102 of the tongue 101. At the same time, the ejector 140 is pushed and moved by the head of the tongue 101 and thrust into the stopper member 150 by resisting being biased by the coil spring 145. In this way, the tongue 101 is engaged with the connector buckle 100.
On the other hand, disengagement of the connector buckle 100 and the tongue 101 is done by inserting a member such as a vehicle key into a cutout 162 formed in the cover member 160b to push and move the unlocking member 130 in a manner to slide the latch member 120 in the second sliding direction X2 to thereby release the latch member 120 from the locking groove 102 of the tongue 101. When such a pushing/moving operation is given, the ejector 140 pushes out the tongue 101 in the releasing direction E by being biased by the coil spring 145, and the tongue 101 is released from the connector buckle 100.
The connector buckle 100 has the problems described below.
First, formation of the facing surfaces of the base member 110 is performed by folding the base member 110 at a folding position that is to become the side surface over which the unlocking member 130 is externally fitted. Specifically, sheet-metal working is performed using a sheet metal having a size corresponding to the inner region 111 as an insert member, to fold a flat-plate member to conform to the shape of the sheet metal and thereby form the base member 110 folded to have facing surfaces.
However, in the base member 110 formed in this way, the inner region 111 cannot have a height shorter than the thickness of the sheet metal, which leads to a problem that the base member 110 cannot avoid being thick. A conceivable way to overcome this problem may be to use a thin sheet metal. However, mechanical strength of a thin sheet metal is low, and the sheet metal may be torn during working.
Next, when the ejector 140 is to be assembled on the base member 110, the ejector 140 is tied up with the coil spring 145 and the stopper member 150 beforehand, and in this state, inserted into the inner region 111 of the base member 110. Because the stopper member 150 is to be held at a height that is above the inner region 111, it is necessary to assemble the ejector 140 diagonally into the inner region 111 by holding the ejector 140 at a height lower than the stopper member 150. That is, because the stopper member 150 is supposed to support the ejector 140 by enduring the tongue 101 to be inserted, the stopper member 150 is to be fixed on the base member 110 by a protrusion 153 thereof being inserted into a locking hole 114 formed in the base member 110. Hence, the stopper member 150 is held at a height that is above the inner region 111 by an amount corresponding to the height of the protrusion 153, which makes it necessary to perform the assembly diagonally. It is difficult to perform such diagonal assembly mechanically, which is a cause of reducing the productivity of the connector buckle 100.
Hence, in the current circumstances, there is no satisfactory connector buckle available that has a small and simple structure and has a favorable productivity.